WIP: Reorganize Base Chain docs IA

[ericbrown99]

Summary

• Reorganizes Base Chain docs around current builder/operator jobs and adds a new Chain overview.
• Consolidates Flashblocks content into canonical Chain surfaces for block building, finality, RPC behavior, and node operation.

Absorbs PR #1600 (closes #1600):

• Removes all wss://mainnet-preconf.base.org and wss://sepolia-preconf.base.org references from newFlashblocks.mdx, pendingLogs.mdx, and base-networks.mdx
• Updates descriptions and JS examples to direct users to node providers for WebSocket support
• Strips stale Flashblocks WSS and Standard WSS columns from the snippet

Content fixes:

• Restores ecosystem-contracts.mdx (accidentally dropped by the IA reorg)
• Distributes all Flashblocks FAQ content from the removed FAQ page into transaction-ordering.mdx, flashblocks-api-overview.mdx, and troubleshooting-transactions.mdx
• Adds Wagmi, Viem, and Ethers library examples to flashblocks-api-overview.mdx
• Updates stale "Azul activates" language to "Azul activated" throughout

RPC overview improvements:

• Removes the duplicate "Endpoint URLs" accordion (content lives on connecting-to-base)
• Standardizes all card groups to use no icons (removes inconsistency where only some cards rendered icons)
• Tightens card descriptions to one sentence each

Specs:

• Updates specs/overview.mdx lineage section to reference Azul as the Base Stack transition point
• Removes stale Azul upgrade guide link from specs/protocol/overview.mdx

Rationale

This keeps the recent Chain docs work coherent as a single reviewable change: navigation, redirects, factual network references, API behavior, and AI-facing indexes all move together instead of landing in a partially inconsistent state.

Validation

• Rebased onto latest origin/master.
• Ran stale literal/link QA sweep for the Chain docs edits.
• Ran node scripts/lint-mdx.js; it fails repo-wide on existing unrelated issues. The only PR-adjacent lint item is docs/snippets/base-networks.mdx missing frontmatter/no heading, matching existing imported-snippet patterns.

[cb-heimdall]

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